In the typical industrial uses of vortex tubes, compressed air is supplied as the source of power. Such air is usually pressurized in the 80 to 100 psig range and, although filtered, is not subject to any special drying procedures. As a result, compressed air entering such a vortex tube is usually saturated with water vapor in an amount equal to the saturation level for the temperature and pressure of the compressed air supply.
Within such a vortex tube, the compressed air is throttled through nozzles and lowered to approximately atmospheric pressure. (For a discussion of counterflow vortex tubes and their method of operation, reference may be had to Fulton U.S. Pat. Nos. 3,173,273 and 3,208,229, and Ranque patent 1,952,281.) As a result of the throttling process, the air spins very rapidly and undergoes special temperature change effects which are the unique characteristics of a vortex tube. Usually a vortex tube is used for the cold air produced, and in most cases approximately 60% of the air will exit from the cold air outlet of the tube. This air, having lost its pressure, undergoes a temperature change and leaves the vortex tube at very low temperatures. Typical temperatures range from minus 40.degree. F. to plus 20.degree. F.
The first process, that of lowering pressure, tends to increase the capacity of the air to hold water vapor. Therefore, the compressed air which enters the vortex tube at 100% relative humidity (a saturated condition) leaves the nozzles at less than 100% relative humidity. The second process, that of cooling the air, tends to reduce the capacity of the air to hold moisture.
In the vast majority of all vortex tube applications, the second effect is stronger than the first effect. Therefore, the net result of the two processes (lowering pressure and then lowering temperature) is to reduce the ability of the compressed air to hold moisture. Because of this situation, moisture is nearly always condensed in conventional vortex tube applications and, because the exit temperature of the cold air is usually well below 32.degree. F., that condensed moisture appears not as a liquid but as a finely-divided snow or ice.
In many vortex tube applications, the cold air must travel through associated elements or equipment before it is used. In particular, since the high-velocity cold air stream discharged from a vortex tube frequently exhibits a raucous, unpleasant noise, sometimes even a screeching or whistling sound, efforts have been made to provide sound-suppressing mufflers which may be coupled to the cold air outlets of such tubes. Unfortunately, conventional muffler designs are at best only partially effective, not because they are incapable of suppressing noise but because they tend to become clogged with ice, thereby blocking the continued flow of cold air. If, for example, a glass fiber muffler were used with a vortex tube having a cold air discharge temperature well below 32.degree. F., the fine ice content in the cold air would tend to block the small passages in the packed muffler, ultimately freezing into a solid mass which might totally obstruct the flow of cold air. While mufflers with straight-through passages, some having re-entrant tubes, reflecting chambers, and the like, may be less susceptible to icing and clogging, they are less effective than packed mufflers in suppressing noise. Where a vortex tube requires continuous or extended use, or where the cold air fraction discharged from the tube is at the lower part of the typical range given above, even a straight-through muffler may become blocked with frozen moisture.
In some vortex tube applications where muffler icing would be expected to occur, one solution has been to install a central air dryer for removing moisture from the compressed air supplied to the vortex tubes. Such a system is expensive not only to acquire but also to maintain, with the result that some of the advantages of utilizing vortex tubes as industrial cooling devices may be substantially offset. Another approach, especially for use in plants without central compressed air dryers, is to equip the air supply lines to the vortex tubes with antifreeze injectors. (See "Cold Air Coolant Systems," (a technical brochure), p. 4, 1976, Vortec Corporation.) An antifreeze such as ethylene glycol is injected into the air stream to produce an antifreeze mist. While such a mist is effective in preventing icing of the muffler-equipped vortex tubes, the inclusion of an antifreeze injector in the system adds a further complexity and, more importantly, would be unacceptable in those instances where even traces of antifreeze on the work product would be objectionable.